Method for the fabrication of corrosion resistant electrodes

ABSTRACT

An electrode for use in instruments capable of measuring the electrophoretic mobility of particles in solution is disclosed. The electrode is comprised of an inexpensive support member, generally made of titanium, onto a flat surface of which has been connected, generally by microwelding, a flat electrically conductive but chemically inert foil member, preferably platinum. A uniform texture can be generated on the exposed surfaces of the electrode by various means including tumbling the electrode with an abrasive. An oxide layer can be generated on the support member by soaking the composite electrode in an appropriate medium, protecting the exposed surface of the support member from fluid contact with the sample solution, while the foil member, unaffected by the oxidation process, is able to contact the sample solution.

PRIORITY

This application is a division of U.S. patent application Ser. No.14/169,008 filed Jan. 30, 2014, “Corrosion resistant electrodes forelectrophoretic mobility measurements and method for their fabrication,”which claims priority to U.S. Provisional Patent Application No.61/759,207 filed Jan. 31, 2013.

RELATED APPLICATIONS AND PATENTS

The following patent applications relate to the measurement of theelectrophoretic mobility of particles and are hereby incorporated byreference:

U.S. Pat. No. 8,441,638 B2, H.-T. Hsieh and S. Trainoff, “Apparatus tomeasure particle mobility in solution with scattered and unscatteredlight,” issued May 14, 2013.

U.S. Pat. No. 8,525,991 B2, H.-T. Hsieh and S. Trainoff, “Method tomeasure particle mobility in solution with scattered and unscatteredlight,” issued Sep. 3, 2013.

P.C.T. Application PCT/US12/49641, S. Trainoff, “Method and apparatus tosuppress bubbles in optical measurement cells,” filed Aug. 5, 2012, andclaiming priority to U.S. Provisional Application 61/515,796, filed Aug.5, 2011.

BACKGROUND

The present invention involves an innovative electrode design generallyfor use in instruments that measure the electrophoretic mobility ofmacromolecules in solution wherein the charged particles within solutionare subject to an applied electric field, and their resulting motion ismeasured. Although the present disclosure will refer to macromoleculesthroughout much of its description, measurements using the inventiveapparatus disclosed herein may include, more generally, all classes ofsmall particles including emulsions, viruses, nanoparticles, liposomes,proteins, macro-ions, and any other solution constituents whose size maylie between about a half and a few thousand nanometers. Thus whenever aterm such as “molecule,” “macromolecule,” “particle,” or “macro-ion” isused, it should be understood to include all of the aforementionedsolution-borne objects to be subject to some form of opticalmeasurement.

Electrophoretic mobility is the directly measurable and most widely usedquantity to characterize the charge of molecules, or other particles insolution. Once measured, the electrophoretic mobility can be used inturn to determine the effective charge, Ze, carried by such molecules aswell as their so-called zeta potential. The interface between the groupof ions tightly bound to the particle and those of the surroundingsolution that do not move with the particle defines the hydrodynamicshear plane. The zeta potential represents the electrostatic potentialexisting at this shear plane. It is an objective of the presentinvention to improve the reliability of measurements of electrophoreticmobility, effective charge, and zeta potential of molecules andparticles in solution contained within a measurement cell, as well as toimprove the durability of the instruments and their components.

Several techniques have been developed and are available for measuringmobilities including light scattering methods such as heterodyne DLSincluding both laser Doppler electrophoresis, LDE, and phase analysislight scattering, PALS. These techniques involve measuring lightscattered from moving particles, whereby such scattered light carriesinformation relating to such motion and from which the associatedelectrophoretic mobility of the particles may be determined.

Instruments that measure electrophoretic mobility must, by necessity,apply an electrical field, generally between two electrodes, in a fluidsample to induce electrophoresis. The resulting motion is generallyprobed optically to determine the resulting sample velocity. Thiscompromises a first principles measurement of mobility, which is wellestablished as an important parameter for predicting the stability ofcolloidal suspensions. In recent years electrophoretic mobility isfinding new use in determining the stability of molecular solutions.Over the years, many electrode designs have been used. An objective ofthe present invention is to provide an inexpensive electrode thatapplies a uniform field and is mechanically and chemically durable.

A BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the front and side view of an exemplar optical cell formeasurement of electrophoretic mobility.

FIG. 2 illustrates the elements of an embodiment of the invention with aplatinum foil member to be microwelded onto a titanium support member.

A DETAILED DESCRIPTION OF THE INVENTION

As discussed above, electrophoretic mobility measurement instrumentsgenerally employ two electrodes comprising planar surfaces placedparallel to each other, between which is placed a liquid sample andacross which an electric field is generated. The applied electric fieldinduces electrophoresis within the fluid sample. A graphicrepresentation of the elements of an optical electrophoretic mobilitymeasurement cell 100 is shown in FIG. 1. A beam of light 101, typicallygenerated by a laser, passes through an optically transparent window 102and into a sample chamber 103, wherein a fluid sample has been placed,generally through injection ports within the top plate 105. The beamleaves the chamber through exit window 104, and the physical propertiesof the sample molecules are derived based on measurements of theemerging beam and light scattered therefrom. The necessary electricfield is generated between electrodes 106. It should be noted that manyvariations on this optical measurement cell exist, and the utility ofthe present invention is not limited to use with only a flow throughsystem such as that shown in FIG. 1. One example of a measurementchamber for which the present invention may be advantageous is a samplecuvette containing the necessary elements of the measurement cell. Themeasurement chamber of the cuvette may be filled with sample prior tothe cuvette being placed within the instrument itself, and thereby thesample is placed in the path of the beam. While the particular elementsof the instruments and measurement cells with which the presentinvention may be utilized may vary, this disclosure is concernedprimarily with the electrodes used to generate the electric field withinthese cells.

For many years platinum has been the preferred material for electrodemanufacture because it is chemically inert, even in high salt buffers,and does not oxidize. Pure platinum however, is very expensive so thereis a strong incentive to minimize the amount of material that is used. Acommon strategy is to fabricate the electrodes out of an inexpensivematerial, such as stainless steel, and then protect them with anelectroplated platinum coating. This technique is effective, but suffersfrom a number of problems. In order to get good adhesion, the substratemust be exceptionally clean. Even a small amount of contamination on thesurface or in the plating baths can give rise to coatings that crack orflake off. Even when extreme caution is taken to insure good adhesion,the resulting platinum coating is brittle. If the coating is made morethan about 2-5 μm thick, the mechanical strain that develops during thecoating process frequently causes the film to crack, exposing theunderlying substrate. If the coating is made less than 2 μm thick, itmay be porous, again exposing the substrate. Also, when an electricalfield is applied, the underlying substrate can corrode causing theplated surface to loosen. Further, when the electrodes are cleaned,either chemically, or by gentle mechanical abrasion, the coating candetach from the substrate surface. It is therefore of criticalimportance that plated surfaces, with their attendant chemical andmechanical problems, be avoided in the fabrication of electrodes.

The present invention involves an innovative electrode design and itsmethod of manufacture, which, in its various embodiments, offers theadvantages of a planar surface in contact with the sample fluid which isless prone corrosion than electrodes created by standard electroplatingtechniques. In one embodiment this is achieved by microwelding aplatinum foil surface to a support made of a less expensive materialsuch as titanium. More specialized embodiments aid in the prevention ofbubble formation on the electrode surfaces and limit the surfaces of theelectrode in contact with the fluid to the planar electrode surface.

In one embodiment of the invention whose elements are shown in FIG. 2,the electrode 200 is comprised of a support member 201 made of anappropriate, but relatively inexpensive material such as titanium ontowhich is welded a disc of bulk conductive foil 202. This microweldingaround the perimeter of the platinum foil may be performed by anelectron beam. The use of electron beams to microweld surfaces is wellknown in the art. An o-ring groove 203 and other physical elements maybe present as part of the support member. While platinum is a preferredmaterial for this foil surface, the invention should not be limitedthereto. Other chemically inert but electrically conductive materialssuch as gold or various alloys capable of being microwelded to a supportmember may also be used. As regards to platinum, or other expensivematerials, it is clear that the cost of a thin foil microwelded to aninexpensive support member is far less than that of an electrodecomposed of solid platinum. The inventive design thus offers aconsiderable cost savings while still providing the benefits of aplatinum electrode.

The foil surface itself can be made as thick as desired to insuremechanical robustness while still minimizing the amount of materialconsumed. In a preferred embodiment, the platinum foil is roughly 100 μmthick, insuring that it can be cleaned with mild abrasives withoutdamage.

One limitation of using bulk foils is that they are generally formed byextrusion through rollers giving rise to a smooth surface with smallscratches. These scratches can act as nucleation sites for electrolysisbubbles that are often formed during the measurement of samples in highconductivity solutions. To minimize the formation of bubbles, a uniformsurface texture is preferred. This limitation can be overcome withanother embodiment of the invention, wherein the electrode is etchedafter welding. Several methods can be used to achieve the desiredsurface etching. For example, the application hydrofluoric acid or ionbeam etching may be employed. In a preferred embodiment, the etching isachieved by mechanical tumbling with an abrasive. An advantage ofmechanical tumbling is that it is possible to adjust the tumbling timeand abrasive bead size to easily control the final surface finish.

One limitation of some embodiments of the invention described above isthat both the platinum foil and titanium support member are in contactwith the fluid sample. In order to apply a uniform field, it isdesirable that only the parallel plates formed by the platinum foil arein contact with the sample. This can be achieved with another embodimentof the invention wherein the composite electrode is soaked in a strongoxidizer that causes an oxide layer to grow on the exposed surfaces ofthe support member but wherein the platinum foil is unaffected. Apreferred method for achieving this oxidation layer on the supportmember is the soaking of the composite electrode in a solution of 30%hydrogen peroxide in water. The oxide layer may additionally begenerated with other oxidizers or electrochemically, by passing currentthrough the electrode while bathed in a salt solution. The resultingelectrode is inexpensive, chemically inert, and mechanically robust.

As will be evident to those skilled in the arts of materials science andoptical and electrophoretic mobility measurements, there are manyobvious variations of the methods and devices of the invention andmethod for manufacture thereof that do not depart from the fundamentalelements that disclosed herein; all such variations are but obviousimplementations of the described invention and are included by referenceto our claims, which follow.

The invention claimed is:
 1. A method comprising: providing anelectrically conductive support member comprising at least one planarsurface; providing a flat, chemically inert, electrically conductivefoil member; welding the foil member to the conductive support memberabout the perimeter of the foil member, forming, thereby, a compositeelectrode, wherein the welding is performed by an electron beam; etchingan exposed surface of the foil member of the composite electrode so asto achieve a uniform, non-smooth surface there upon, wherein the etchingcomprises mechanically tumbling the composite electrode with anabrasive; and in response to the etching, exposing the compositeelectrode to an oxidation agent, thereby causing an oxide layer to beformed upon an exposed surface of the support member.
 2. The method ofclaim 1 wherein the foil member comprises platinum.
 3. The method ofclaim 2 wherein the foil member is a disc of platinum foil.
 4. Themethod of claim 3 further comprising milling an o-ring groove about aperimeter of the support member wherein the perimeter of the supportmember is chosen such that the perimeter does not intersect the planarsurface to which the foil member is attached.
 5. The method of claim 2wherein the foil member is 100±5 μm thick, permitting thereby the foilmember to be cleaned with mild abrasives without damage.
 6. The methodof claim 1 further comprising: selecting the support member to becomposed of a material or materials whose surface, when exposed to astrong oxidizer, causes an oxide layer to grow thereupon; and selectingthe foil member to be composed of a material or materials whose surface,when exposed to a strong oxidizer, resists an oxide layer formingthereupon.
 7. The method of claim 1 wherein the oxidizing agent is asolution comprising hydrogen peroxide and water.
 8. The method of claim7 wherein the solution consists of 30% hydrogen peroxide and 70% water.9. The method of claim 6 further comprising passing an electricalcurrent through the composite electrode while the composite electrode isbathed in a salt solution, resulting in an oxide layer to form upon thesupport member of the composite electrode.